I'm pretty sure only EJ, Sami, Rafe, and Will actually know. I'm sure Kate suspects because she gave Sami the heads up about EJ's plan. But Stefano doesn't know. None of the Brady's know, although I think Bo and Hope had suspicions. And Gabi and Nick don't know. Which is really who I would like to find out. I'd also like Gabi to find out that Arianna knew and then died. I know it's not Sami and Will's fault, but if Sami is going to hurl false accusations, Gabi should get to do the same.

Bo knows. He encouraged Rafe to have a quickie wedding so Rafe wouldn't have to testify against Sami, so he knows about Rafe's involvement, too. Roman, Caroline and Brady also know about the shooting.

Nicole knows. She's the one that got the tape to EJ proving it. And I thought Brady knew about it as a result of that, too but I could be wrong on that.

I think Lucas mentioned knowing about it when he came back to town, too.

Yeah, Nicole is the one who told Brady. And Gabi went through a period of being angry with Sami over what happened to Ari and she suspected that Ari had played a part in Sami having to give EJ custody of the kids, but IDK if she ever connected all the dots to figure out that Sami shot EJ.